1) Field of the Invention
The field of the present invention relates to sound reproduction and, more specifically, to a speaker configuration and related sound processing for use in a sound system.
2) Background
Attaining optimal sound quality in surround sound or multi-channel sound systems, over the largest possible listening area, can be quite challenging. Some of the difficulties in achieving optimal sound quality in such systems result from the fact that a wide variety of different surround sound and multi-channel audio formats and speaker configurations exist, so that a particular sound system may have reasonably acceptable sound with respect to one or perhaps two audio formats yet sub-optimal sound with respect to other audio formats. Therefore, where a consumer desires, for example, to use a single sound system to play sound recordings in a variety of different formats, different levels of sound quality, some of which are poor or impaired, are likely to be experienced. While the user can adjust speaker positioning or relative balances to try to improve sound quality, such techniques may involve significant manual effort or inconvenience, may be hard to reproduce consistently, and may benefit only one or perhaps a few listeners in a relatively small portion of the listening area.
Existing surround sound recording formats include those referred to as 5.1, 6.1 and 7.1. The 5.1 surround format comprises a compressed data stream containing five channels, generally designated left, center, right, surround left, and surround right, named for the speaker positions for which the channel information is intended. A low frequency effects channel is formed by a combination of the five other channels, and may be provided to a sub-woofer. The 6.1 surround format includes the same five channels as the 5.1 surround format, but adds a surround back channel, which may be fed to one or more back speakers in a surround sound system. The 7.1 surround format is similar to the 6.1 surround format, but has two surround back channels (surround back left and surround back right) rather than a single back channel, for a total of seven channels. Thus, the 5.1 surround format has two surround channels (surround left and right), the 6.1 surround format has three surround channels (surround left, right and back), and the 7.1 surround format has four surround channels (surround left and right, and surround back left and right).
Basic surround system speaker configurations generally include from six to eight speakers placed at conventionally well-established locations, according to the type of surround format they are intended to play. A six-speaker surround system typically includes left, right and center speakers (with the right and left speakers spaced widely apart), a sub-woofer, and surround left and right speakers (which may be monopolar or dipolar in nature). A seven-speaker surround system typically includes the same speaker arrangement as the six-speaker surround system, but adds a back surround speaker. An eight-speaker surround system typically includes the same speaker arrangement as the six-speaker surround system, but adds a back left surround speaker and a back right surround speaker.
The enjoyment experienced by a listener in a surround sound system can be affected by a number of factors, including the listener's physical position relative to the various speakers, as well as by the particular format of the audio track being played on the system. For example, when a 5.1 surround format soundtrack is played on an eight-speaker (7.1) surround system, certain anomalies may occur. An example is that, if the 5.1 surround left and surround right audio signals are monaural, then the left and right surround effects can disappear, being replaced by a single central “phantom” sound image at the rear. Another phenomenon is that if the listener is positioned in the middle of the surround left and surround right speakers, he or she may perceive the surround left and right sound (if monaural) to be higher in volume that it otherwise would be, primarily due to the additive effect of the sound waves intersecting at the listener's position (known as a “lift” effect). If the sound pans from one side to the other (e.g., from left to right), the sound volume may appear to increase as left/right balance is achieved, and then appear to decrease as the sound continues to pan, even though the audio output level remains constant, due to the same “lift” effect. The sound quality may also seem to be “unstable,” in the sense that if the listener moves from the center position, the sound might seem to “flip” from one side to the other.
Some of these effects can be mitigated in 5.1 surround sound systems by the use of adaptive decorrelation with respect to the surround left and right speakers, which derives two substantially decorrelated signals when the surround left and right signals are monaural, in order to provide an improved enveloping surround effect.
When a 6.1 surround format soundtrack is played on an eight-speaker (7.1) surround system, certain other anomalies may be experienced. Since the two rear surround speakers (left and right) are each fed with an identical monaural signal (that is, the same surround back signal), a centrally located “phantom” image may result when the listener is positioned approximately equidistant from the speakers. Reported side effects of this arrangement include “coloration” associated with the phantom image (for example, the sound may seem “unnatural”), a narrow “sweet spot” due to lack of sound image stability when the listener moves off center, and a comb filter effect (in other words, nulls may be produced due to sound wave cancellation effects).
Besides surround systems, a variety of multi-channel recording and playback systems also exist. Examples of some common multi-channel sound systems are Dolby AC-3, DTS, and DVD-Audio, each of which has its own specific digital encoding format. Unlike cinema sound, there is generally no single adopted standard of either loudspeaker type (e.g., full range, satellite plus sub-woofer, dipole, monopole) or speaker layout for most multi-channel audio formats. Any user therefore desiring to listen to multi-channel soundtracks, and/or any of the surround formats (5.1, 6.1 and 7.1), is required either to accept one speaker layout optimized for a particular audio format and experience a compromised performance for all others, or to reconnect various speakers to suit the audio format a particular soundtrack.
Beyond the surround sound environment, other sound systems also face similar challenges, such as attaining a suitably wide “sweet spot” in which the perceived area and stability of a stereo sound image is maximized. In most traditional sound systems, the convention has been to place left and right speakers far apart physically, under the theory that the human ear is thereby better able to perceive the richness of the audio subject matter. However, under many left/right speaker configurations, the sound at off-axis listening positions may be sub-optimal. The quality of sound at a given off-axis listening position may be affected not only by the difference between left and right volumes resulting from the different distances to the left and right speakers, but also by the slight difference in time it takes the aural information to reach the listener.
Accordingly, it would be advantageous to provide an improved sound system which overcomes one or more of the foregoing problems or shortcomings.